Answer
$a_5=60$
$a_6=73$
$a_7=86$
$S_7=329$
Work Step by Step
$a_1=8=8+13(0)=8+13(1-1)$
$a_2=21=8+13(1)=8+13(2-1)$
$a_3=34=8+13(2)=8+13(3-1)$
$a_4=47=8+13(3)=8+13(4-1)$
That is, $a_n=8+13(n-1)~~$ (Arithmetic Sequence)
$a_5=8+13(5-1)=8+13(4)=60$
$a_6=8+13(6-1)=8+13(5)=73$
$a_7=8+13(7-1)=8+13(6)=86$
$S_7=\frac{n}{2}(a_1+a_n)=\frac{7}{2}(8+86)=329$
